1. Field of the Invention
The present invention relates to a method of forming metal bumps such as solder bumps or the like and an electronic device having such bumps.
2. Description of the Related Art
In information processing systems for processing vast amounts of information at high speed, LSIs and VLSIs having electronic circuits and electronic parts integrated on a semiconductor chip find wide applications. In order to mount a semiconductor chip having integrated electronic circuits to a ceramic substrate, for example, metal bumps of solder or the like are formed on the semiconductor chip or the ceramic substrate, and the semiconductor chip is mechanically coupled with fixed on and electrically connected to the ceramic substrate, by fusing the solder bumps. For this purpose, it is necessary to form solder bumps on the semiconductor chip or the ceramic substrate or printed circuit board in advance.
Japanese Patent Publications JP-A-7-249631 and JP-A-9-36118 disclose a method of forming solder bumps on an electronic member such as a semiconductor chip, using a bump-forming plate having a plurality of cavities. In this method, a plurality of cavities are formed in the surface of a flat bump-forming plate, the cavities of the bump-forming plate are filled with solder paste by squeegeeing, the bump-forming plate is heated thereby to form the solder balls from the solder paste in the cavities, and the semiconductor chip is moved toward the bump-forming plate thereby to transfer the solder balls from the plate to the semiconductor chip.
As a result of filling the cavities of the bump-forming plate with solder paste by squeegeeing, the surface of the solder paste is flush with the surface of the bump-forming plate, so that a predetermined amount of solder paste is inserted in each of the cavities. The solder component of the solder paste in the cavities of the bump-forming plate is heated and molten, and is rounded by surface tension to form solder balls. Each of the solder balls, while being held in each cavity of the bump-forming plate, has the top thereof protruded upward out of the surface of the bump-forming plate. With the movement of the semiconductor chip toward the bump-forming plate, therefore, electrode pads on the surface of the semiconductor chip come into contact with the tops of the solder balls, with a small gap left between the surface of the semiconductor chip and the surface of the bump-forming plate. In this way, the solder balls can be transferred from the bump-forming plate to the semiconductor chip.
According to this method, the solder bumps are exactly arranged in accordance with the position of the cavities of the bump-forming plate, and solder balls are formed in a uniform size corresponding to the amount of the paste (i.e. the size of each cavity) inserted in the cavities of the plate, thus making it possible to form bumps with high accuracy and low cost. Especially in the case where a bump-forming plate of silicon is used and cavities are formed in the silicon plate by anisotropic etching, a multiplicity of minuscule cavities can be accurately formed at small pitches. This method can thus be suitably used for forming bumps on a semiconductor chip having a high-density wiring at small pitches.
However, the higher the density and hence the smaller the pitches of the wiring of the semiconductor chip, the smaller the solder bumps to be formed, and hence the smaller the extent of the solder ball tops which are protruded from the surface of the bump-forming plate. If a small distortion of the surface of the bump-forming plate or the semiconductor chip exists, a very small proportion of the solder balls are not transferred from the bump-forming plate to the semiconductor chip and may cause a defect.
Also, if the gap between the surface of the semiconductor chip and the surface of the plate becomes smaller at the time of transfer of the solder balls, foreign matters may intrude between the bump-forming plate and the semiconductor chip and may be pressed to the semiconductor chip, with the result that the surface of the semiconductor chip may be damaged or the foreign matters may be undesirably attached to the wiring of the semiconductor chip.
It is therefore desirable to permit the tops of the solder balls to upwardly protrude to a comparatively large measure from the surface of the bump-forming plate, even if the size of the solder balls are reduced.
Further, in operating an electronic device comprising a semiconductor chip and a circuit board coupled to each other by bumps, heat is generated in the electronic circuits of the electronic device and the semiconductor chip and the circuit board are deformed. The difference in coefficient of thermal expansion between the semiconductor chip and the circuit board causes a difference between the amount of deformation of the semiconductor chip and that of the circuit board, thereby causing a stress in the metal bumps coupling the semiconductor chip and the circuit board to each other. Repetitive exertion of stress on the metal bumps progressively accumulates the fatigue of the metal bumps and leads to the problem of a reduced durability or reliability of at least a portion of the metal bumps. The stress exerted on the metal bumps is comparatively low in the central area of the circuit board and comparatively high in the outer peripheral area thereof. The metal bumps located in the outer peripheral area, therefore, tend to be reduced in durability and reliability.
The object of the present invention is to provide a method of forming small bumps by which metal balls protrude to a greater extent with respect to the size thereof, and thus smaller bumps can be securely formed and an electronic apparatus including a device having the bumps formed by such a method.
Another object of the present invention is to provide a bump-forming method and an electronic device by which the durability and reliability of metal bumps can be improved.
A bump-forming method, according to the present invention, comprises the steps of filling cavities of a first plate with metal paste by squeegeeing, filling cavities of a second plate arranged in a mirror symmetry relationship with the cavities of the first plate with metal paste by squeegeeing, setting the first plate and the second plate in a facing relationship to each other with the cavities of the first plate and the cavities of the second plate located in registry with each other; heating the first and second plates to form metal balls in the cavities in one of the first and second plates, moving one of the first and second plates relative to the other of the first and second plates; and transferring the metal balls in the cavities in one of the first and second plates to a device to which bumps are to be formed.
The metal paste is composed of metal powders such as solder particles and flux, for example, and is inserted in the cavities of the first and second plates by squeegeeing. The surface of the solder paste is flush with the surfaces of the first and second plates, respectively, so that a predetermined amount of solder paste is inserted in each of the cavities. Then, the first and second plates are superposed one on the other and heated, with the result that the metal powders in the metal paste are molten and rounded into metal balls.
Since the cavities of the first plate are located in registry with the cavities of the second plate, the molten metal component of the metal paste in each of the cavities of the first plate is merged with the molten metal component of the metal paste in the corresponding one of the cavities of the second plate thereby to form a single metal ball. The metal balls are located in the cavities in one of the first and second plates with the tops thereof protruded out of the cavities, respectively. One plate is moved away from the other. The metal balls located in the cavities in one of the first and second plates are transferred to a device to which bumps to be formed.
According to the present invention, the amount of the metal paste in one of the cavities of the first plate plus the amount of the metal paste in one of the cavities of the second plate is equal to the amount of metal paste required to form one metal ball. Specifically, the total of the volume of a cavity of the first plate and the volume of a cavity of the second plate is equal to the volume required to form one metal ball. The metal balls are finally held in the cavities in one of the first and second plates. Each metal ball thus held has a larger volume than a particular cavity in which it is held. Consequently, each metal ball, while being held in the cavity, has the top thereof protruded considerably from the surface of the plate.
In transferring the metal balls from the plate to a device to which bumps are be formed, the metal balls come into contact with the electrode pads of the device, with a comparatively large gap maintained between the particular plate and the device. In other words, the metal balls can be transferred while maintaining a larger gap between the plate and the device to which bumps are be formed with bumps than that in the prior art.
Therefore, even if the wiring of the device to which bumps are be formed is formed at an increased density with narrower pitches, the metal balls are securely transferred from the plate to the device, while at the same time, damage of the surface of the semiconductor chip and attachment of foreign matter to the wiring of the semiconductor chip are prevented.
Preferably, the first and second plates are superposed one on the other vertically. Metal balls are thus formed in the cavities of the lower plate.
Preferably, before superposing the first and second plates, one of them is heated to form metal balls in the cavities thereof. The plate in which metal balls have not been formed is superposed above the plate in which metal balls were formed, and the assembly is then heated to form merged metal balls.
Preferably, the first and second plates are made of silicon, and the cavities are formed in the silicon plates by anisotropic etching. As an alternative, the first and second plates are made of photosensitive glass with the cavities formed in the photosensitive glass by etching. As another alternative, the first and second plates are formed in such a way that replicas having projection are formed on a plate having cavities by plating, and the first and second plates formed by molding using the replicas.
Preferably, the melting point of the metal powders in the metal paste filled in the cavities of one of the first and second plates is higher than that of the metal powders of the metal paste filled in the cavities of the other plate. As a result, it is possible to form metal balls of a composite structure with a core having a high melting point and an outer peripheral portion of a low melting point.
Further, according to the present invention, there is provided an electronic device including a device having bumps formed by the bump-forming method described above.
According to another aspect of the present invention, there is provided a bump-forming method comprising the steps of preparing a plate having a plurality of cavities, the cavities in the outer peripheral area being smaller than the cavities in the central ones; filling the cavities of the plate with metal paste by squeegeeing; heating the plate to form metal balls in the cavities; and moving the plate toward a first device to which bumps are to be formed to transfer the metal balls in the plate cavities to the first device.
The metal paste is composed of metal powders such as solder powders and flux, for example, and inserted in the plate cavities by squeegeeing. The surface of the solder paste is flush with the surface of the plate, so that the solder paste of an amount equal to the volume of a cavity is filled in each cavity. When the plate is heated, the metal powders in the metal paste are molten into round balls. The metal balls in the plate cavities are transferred to the device to which bumps are to be formed.
Since the cavities in the outer peripheral area of the plate are smaller than the cavities in the central area thereof, the metal balls formed in the cavities in the outer peripheral area are smaller than the metal balls formed in the cavities in the central area. As a result, the bumps formed in the outer peripheral area of the device are smaller than those formed in the central area thereof. A first device (first electronic member) to which bumps are to be formed is coupled to a second device (second electronic member), using the bumps thereby to form an electronic device. The bumps located in the central area are large and pressed to a comparatively large degree, while the bumps located in the outer peripheral area are small and pressed to a comparatively small degree. Consequently, even if stress is exerted on the bumps while the electronic device is in use, the fatigue of the small bumps in the peripheral area is comparatively small. As a result, the durability and reliability of the small bumps located in the peripheral area are improved.
Preferably, the plate is made of silicon and the cavities are formed in the silicon plate by anisotropic etching. As an alternative, the plate is made of the photosensitive glass, and the cavities are formed in the photosensitive glass by etching.
Preferably, the first and second plates are formed in such a way that replicas having projections are formed on a plate having cavities by plating, and the first and second plates are formed by molding using the replicas.
Further, according to the present invention, there is provided an electronic device comprising a first electronic part and a second electronic part coupled together by metal bumps, the metal located in the bumps located in the outer peripheral area being smaller than the metal bumps central area. Also in this case, as in the aforementioned case, the durability and reliability of the bumps located in the peripheral area are improved.
Preferably, the metal bumps in the outer peripheral area are formed in a pincushion shape.